This application is based on Japanese Patent Application No. 11-215905 (1999) filed Jul. 29, 1999, the content of which is incorporated hereinto by reference.
1. Field of the Invention
This invention relates to a wavelength-tunable mode-locked laser, a wavelength converter and a transmission system that are applicable to, for example, optical communication networks utilizing fast wavelength conversion and wavelength routing.
2. Description of the Related Art
(First Example of Prior Art)
For implementation of optical communication networks that utilize the wavelength division multiplexing method, the wavelength converter plays an important role in changing the wavelength of optical signals. Research conducted by S. J. B. Yoo Wavelength Conversion Technologies for WDM Network Applications Journal of Lightwave Technology, Vol. 14, No. 6, pp. 955-66 (June 1996), for example, is part of the associated wide-range investigation. Among many methods for wavelength conversion, the architecture focused on in this invention particularly relates to technology to convert transmitted optical information into an output light of a different wavelength by the use of a wavelength-tunable light source.
There are two types of methods for wavelength conversion for the above purpose: one is a wavelength selection by a network management system; and the other is that conducted by the transmitted optical signal itself.
FIGS. 34 and 35 illustrate typical configurations of wavelength converters of the two methods.
Referring now to FIG. 34, optical signal 15 is guided to a receiver 13 to decode the information signal. By an optical modulator 12, which is driven by this information signal, the output light from a wavelength-tunable light source 11 can be provided as optical signal 16 of which wavelength has been converted.
The wavelength of light emitted from the wavelength-tunable light source 11 is controlled by control signals sent from a network management system 17 via an information processing circuit 14. The information processing circuit 14, interpreting the control information, provides current and voltage which are necessary to change the wavelength of light emitted from the wavelength-tunable light source.
In the case of the wavelength converter shown in FIG. 35, the control signal is directly decoded by the information processing circuit 24, based on the optical signals under transmission. Namely, the optical information signal 25 has both communications information and control information. This control information included in information signals is called xe2x80x9cheaderxe2x80x9d and the signal originator can send information about wavelength conversion to intermediate nodes in the network by the use of this header.
The wavelength conversion described in FIG. 35 is the same as that performed in the wavelength converter of FIG. 34, where a receiver 23 corresponds to the receiver 13, and an optical modulator 22 to the optical modulator 12.
(Second Example of Prior Art)
The time-to-wavelength mapped laser of prior art is a wavelength-tunable mode-locked laser of which oscillation wavelength is changed by the repetition frequency of the resonator. It has several advantages such as simple structure, fast wavelength conversion and easy wavelength selection.
FIG. 37 illustrates general properties of oscillation 150 of the above laser.
Referring to FIG. 37, there are several clock frequencies fi for input clock signals. When a clock signal is applied to laser, a synchronous oscillation (pulse oscillation) occurs at a repetition frequency fi and a wavelength of xcexi.
Since one clock frequency provides one oscillation wavelength, the oscillation frequency of laser is determined by selecting a clock frequency fi of the clock signal.
FIG. 27 is a block diagram illustrating the configuration of the time-to-wavelength mapped mode-locked laser (detail explanation will be provided later).
In FIG. 27, wavelength mapped delay circuit 1-4 in laser resonator 1-1 is a circuit that provides a different propagation delay to light of each wavelength (namely, optical path length). The oscillation characteristics are shown in FIG. 38.
In FIG. 38, there are as many as N wavelengths as input wavelength xcexi. Each wavelength has an intrinsic optical path length xcex94Lopt (xcexi) and corresponding propagation delay xcex94T(xcexi).
When the light of wavelength xcexi enters a time-to-wavelength mapped circuit, it travels a length xcex94Lopt(xcexi) before going out, and the light is given a propagation delay xcex94T(xcexi) corresponding to xcex94Lopt(xcexi). The optical path length is a product of physical length xcex94L(xcexi) and refractive index n, namely, xcex94Lopt(xcexi)=nxcex94L(xcexi). With xe2x80x9ccxe2x80x9d being the speed of light in vacuum, the relationship between propagation delay and optical path length is expressed by xcex94T(xcexi)=xcex94Lopt(xcexi)/c.
In FIG. 27, when a wavelength mapped delay circuit 1-4 is inserted in the laser resonator 1-1, the total optical path length Lopt(xcexi) in the whole laser resonator and the corresponding primary repetition interval T(xcexi) (primary repetition frequency f(T)i=1/T(xcexi)) change depending on each wavelength.
By the modulation conducted in optical modulator 1-3 at the frequency equal to the primary repetition interval (namely, repetition interval T(xcexi)/m, m greater than 0), a mode-locked oscillation occurs at the wavelength xcexi that corresponds to this interval.
Because the other wavelengths are transmitted in the resonator 1-1 at intervals that do not match the modulation interval, oscillation of the other wavelengths is suppressed. In other words, the oscillation wavelength of time-to-wavelength mapped laser is selected by setting the frequency of a clock signal for driving the optical modulator 1-3.
In the laser emission shown in FIG. 27, a driver 1-15 generates driving signal 1-14. A clock signal generator 1-8 in the driver provides clock signal 1-9. One of clock signals of frequencies f1, f2 . . . fN, provided by the clock signal generator 1-8, is selected by a clock signal selecting unit 1-7.
The driving signal 1-14 comprises clock signal 1-9 and DC bias signal 1-12. The DC bias signal 1-12 is necessary for setting the operation point of the optical modulator 1-3. DC bias signal 1-12 is generated by the DC bias signal generator 1-11 and its signal level is adjusted by a DC bias signal adjusting unit 1-10. A synthesizing unit 1-13 combines clock signal 1-9 and DC bias signal 1-12.
(Third Example of Prior Art)
Since light transmission is performed at a standard transmission rate (for example, 155.52 Mbps in STM1), a pulse light source where the repetition frequency is fixed in accordance with the transmission rate or a light source that emits beams of continuous wave light (cw light source) is employed.
FIG. 21A shows an example of transmission signals that are sent by the prior art transmission method employing a pulse light source. There is a one-to-one relation between light pulse and data bit.
(First Example of Problems in Prior Art)
First, one of the disadvantages in prior art will be explained below. In the architecture of prior art, the wavelength converters shown in FIGS. 35 and 36 each require information processing circuit 14 or 24 to select a wavelength.
Signals entered into those circuits are control information that includes information about the wavelength of output light. The control signal is written in binary code, for example. The format of signals required to change the wavelengths of light emitted from the wavelength-tunable light source 11 or 21 depend on each light source.
The wavelength can be controlled by changing the input current and device temperature when a wavelength-tunable semiconductor laser (such as distributed feedback semiconductor laser [G. Soda, Y. Kotaki, H. Ishikawa, S. Yamakoshi, H. Sudo and H. Imai, Stability in Single Longitudinal Mode Operation in GaInAsP/InP Phase-adjusted DFB Lasers, IEEEJ, Quantum Electronics, vol.23, pp. 804-14 (1987)], distributed Bragg reflector semiconductor laser [K. Kondo, M. Kudo, Yamakoshi and K. Wakao, A Tunable Wavelength-Conversion Laser, IEEE, Quantum Electronics, vol.28, pp.1343-1348(1992)], sample grating semiconductor laser [V. Jayaraman, Z. M Chuang and L. A. Coldren, Theory, Design, and Performance of Extended Tuning Range Semiconductor Lasers with Sampled Gratings IEEEJ, Quantum Electronics, pp.1824-1834, (June 1993) is used.
The information processing circuit, therefore, is required to output injection current and voltage, for example, to provide a desired wavelength.
There is another type of a wavelength-tunable light source that oscillates at many different wavelengths: [J. Ishikawa and T. Chikama, Japanese Patent Application Laid-open No. 6-188517 (1994); R. Monnard, C. R. Doerr, C. H. Joyner, M. Zirngibl and L. W., Stulz, Direct Modulation of a Multifrequency Laser up to 16xc3x97622 Mb/s, IEEE Photonics Technology Letters, vol.9, pp.815-817, (June 1997)].
Such light sources have a number of semiconductor laser arrays each of which produces a different wavelength, and the wavelength selection is made by oscillating one of the laser devices that provides the desired wavelength.
In this case, the information processing circuit is required to output injection current, for example, to oscillate a proper laser device.
The extra components such as an information processing circuit make the structure of the wavelength converter more complex and cause the cost to be increased.
Further, in the case of a wavelength converter shown in FIG. 36, the header has to be decoded and processed very fast, because the control information is part of transmitted optical signals. This header processing could be accomplished by a special IC (logic circuit) but it is difficult to manufacture such IC at present.
(Second Example of Problems in Prior Art)
Next, a second example of problems in prior art will be explained below.
Referring now to FIG. 27, the prior art time-to-wavelength mapped laser employs an optical fiber of which wavelength dispersion is high or a distributed Bragg grating (DBG) as a wavelength mapped delay circuit 1-4.
This structure employing these components, however, has following problems.
First, when using an optical fiber, a long fiber (several tens to several hundred meters) is required to provide a delay large enough for operation, because the dispersion per unit length of optical fiber is small. Then it becomes difficult to stabilize the operation of laser resonator 1-1. Also since the wavelength conversion rate is proportional to the length of the resonator, it is difficult to realize a fast conversion.
When using a distributed Bragg grating (DBG), it is possible to make the equipment compact. But, in turn, it becomes difficult to provide an arbitrary wavelength interval and propagation delay since the reflection band of the distributed Bragg grating is inversely proportional to its physical length.
In particular, when a short wavelength interval (for example, 50 GHz or 100 GHz) is required, the physical length of each DBG for a given wavelength becomes 5 mm or longer. Then the whole length of DBG becomes very large and the system becomes difficult to design.
In addition, both methods have a common problem that the frequency of clock signal cannot be arbitrarily controlled with oscillation wavelength because the amount of propagation delay for each wavelength cannot be controlled separately.
(Third Example of Problems in Prior Art)
Since the repetition frequency of time-to-wavelength mapped laser changes with wavelength, only wavelength that has the same repetition frequency as the transmission rate is usable in the transmission system of prior art. To solve this problem, some methods have been presented (K. Tamura and M. Nakazawa, Dispersion-tuned harmonically mode-locked fiber ring laser for self-synchronization to an external clock, Optics Letters, vol. 21, pp. 1984-1986(1996); S. Li, K. T., Chan and C. Lou, Wavelength-tunable picosecond pulses generated from stable self-seeded gain-switched laser diode with linearly chirped fibre Bragg grating, Electronics Letters, vol. 34, No. 12, p. 1234-1236(1998); and K. Chan and C. Shu, Electrical switching of wavelength in actively modelocked fibre laser incorporating fibre Bragg gratings Electronics Letters, vol.36, No.1, p.42-43(2000)).
Those proposals, however, have problems such as complex structure of laser equipment, increased cost and degradation of performance, because they require, for example, that the length of resonator has to be variable and that two optical modulators are necessary.
Accordingly, it is an object of the present invention to provide a wavelength-tunable mode-locked laser that can generate output signal having an oscillation wavelength which has an arbitrary wavelength interval independent of the wavelength of the clock signal and a different propagation delay for each wavelength.
Another object of the present invention is to provide a transmission system that conducts transmission at a constant transmission rate, with the repetition frequency of time-to-wavelength mapped mode-locked laser being changed.
Still another object of the present invention is to provide a wavelength-tunable mode-locked laser and a transmission system that can easily decode control signals from optical information signals by the use of a simple configuration of equipment that eliminates the need for complex information processing circuits.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.